Tanning



Patented June 25, 1940 PATENT OFFICE TANNING George D. Graves,Wilmington, DeL, assignor to E. I. du Pont de Nemours 85 Company,Wilmington, Del., a corporation 01' Delaware No Drawing. ApplicationJune 16, 1938, Serial No. 214,051. Renewed September 9, 1939 14 Claims.

This invention relates to the production of leather and is morespecifically directed to processes employing an acidic polymerizationproduct of methacrylic acid for tanning, and to the leather produced.When the term methacrylic" is used herein, the intended meaning isalphamethacrylic." The word acidic, when used in connection withmethacrylic'acid polymers, refers of course to carboxyl acidity.

ill The conversion of skins into leather is accompanied by profoundchanges in their physical characteristics. They become opaque and adistinct fibrous structure becomes apparent. The stiiinesscharacteristic ofdry rawhides disappears to a great extent and theybecome comparatively flexible. They assume a characteristic attractiveappearance and soft feel. Tanning also renders the productimputrescible. For useful tanning these changes must be relativelypermanent, and the leather should be resistant to de-tanning by water,at least at ordinary temperatures.

The ancient and well-developed art of tanning customarily effects theconversion of skins into leather by the use of vegetable preparationsthat contain tannins. Vegetable tanning materials,

such as quebracho extract, are quite slow in their action and tanningprocesses employing them require rather excessively long periods of timefor completion. The leather produced using vegetable tanning materialsis characteristically colored, and it is difficult to obtain good, truecolors upon dyeing. It is particularly dimcult to produce satisfactorylight-colored leather.

Tanning processes employing metal compounds, such as chromium sulfate,are also known. Such processes are, however, subject to a number ofdisadvantages in operation, and the leather produced ischaracteristically colored. White leather, so-called, can be produced byemploying bleaching if required, but this leather is not truly whitesince it has a distinct blue, yellow, green, or gray tint.

Numerous synthetic tanning materials have been proposed, but these aremarkedly inferior to the vegetable or metal compound tanning agentsmentioned. In some instances the leathers produced with known synthetictanning agents are not sufliciently resistant to de-tanning by water,and in other cases the leather lacks the necessary fullness" andplumpness."

Therefore, these synthetic tanning materials of the prior art arecustomarily used in conjunction with a vegetable tanning material or ametal compound tanning agent, and they find their principal usefulnessbecause of properties apart 5 from their characteristics as tanningagents.

It is an object of this invention to provide a fundamentally new type oftanning process. It ,is a further object to provide processes whichoffer a rapid, simple, and easily controlled method w for the tanning ofskins and hides. It is a still further object of this invention toprovide tanning processes by means of which there may be produced a goodquality of leather. It is another object of this invention to producewhite leather. 1

Still further objects will become apparent herein- 5 after.

These and other objects of my invention are attained by the use ofacidic polymerization products of alpha-methacrylic acid as tanningagents. According to the preferred processes of 20 this invention, anacidic polymerization product of methacrylic acid is introduced into askin and then the combination of the skin with the polymerizationproduct is accelerated by lowering the pH of the system. The leather soproduced is white to the extent of having no noticeable tint, and, whencompared with the off-color leather produced by the prior art methods,it is a very true white. The leather is additionally of good character,being opaque, soft, full, and flexible.

Acidic polymerization products of methacrylic acid apparently form somekind of chemical complex, probably thru their free carboxyl groups, 35with skins or hides, and I believe that the leather produced accordingto this invention is a reaction product of the methacrylic acid polymerand skin substance.

The acidic polymerization products of meth- 40 acrylic acid used astanning agents according to my invention may be any of a rather largeclass of products. The term polymerization" is used herein in itsgeneric sense to include polymerization of methacrylic acid with itselfas well as 45 interpolymerization of methacrylic acid with othermaterials. It is necessary only that the polymer used contain asubstantial proportion of free carboxylic acid groups.

The preferred tanning agent of this invention 50 is the .polymer ofmethacrylic acid. hereinafter called :polymethacryiic acid. However,interpolymeis of methacrylic acid with any of a wide variety of otherpolymerizable compounds may advantageously be used as tanning agentsaccording to the present invention. There is no particuiar restrictionon the type of added polymerizable material except that it be one whichdoes not destroy the free acidity of the final polymer to which Ibelieve the tanning eflect is probably attributable.

The present tanning agents are preferably used in aqueous media. Thepreferred agents are water-soluble initially, as is polymethacrylicacid, or are capable of beingbrought into solution with aqueous alkalimetal hydroxides, amines, or other bases. For best results thesolubility in either instance should be such that the solution may bebrought to the proper pH for tanning without precipitation of thetanning agent. In some instances, however, agents. that are moderatelyor even only slightly soluble can be used with some degree of success,due possibly to the fact that, as the small amount in solution is usedup, a small additional amount of the polymer in turn goes into solution,and so on.

The polymerization products of methacrylic acid may be used as tanningagents for any type of skin adapted to be converted into leather or fur.It is to be observed that the term skin is used generically to includeheavy skins such as cowhide, lighter skins such as goatskin andcalfskin, and also skins of fur-bearing animals.

It will naturally be necessary to adapt the tanning processes to thetype of skin to be treated. Goatskin may be more rapidly tanned thancalfskin, but those skilled in the art are well aware of such factorsand may readily adapt the tanning agents of this invention to thespecific needs of particular specialized problems.

Hides to be tanned according to the processes of this invention may beprepared in any of the manners customary in the art. They may suitablybe unhaired, limed, delimed and bated, and pickled. The skins may bede-pickled. They may also be given a salt liquor treatment to softenthem and ,to render them of more uniform condition asis the practice inprior art tanning processes.

While skins prepared in any manner may be treated according to theprocesses of this invention, the pH of the skins is important. As willbe observed below, the tanning is effected at certain conditions ofacidity and the skins may tend to change these conditions. The tanningsolution may suitably be controlled to compensate for any variance ofthe pH of the skins from that of the tanning solutions. It may sometimesbe found advantageous to adapt the processes for preparing the skins sothat a skin prepared for tanning is approximately at the pH of the tanning solution.

Too rapid reaction of the tanning agent with the skin in the firststages of the process should be avoided in order to prevent surfacehardening of the skin. In tanning with the agents of the presentinvention this result is accomplished by careful adjustment of the pH ofthe tanning solution. While the exact acidity of a tanning solutionaccording to this invention may be considerably varied depending uponthe specific type of skin to be treated and depending upon the specifictype of leather to be produced, it is generally desirable to have thetanning solution initially at a pH from about 4.8 to 5.7. Under mostconditions of operation it will be found that a pH of about 5 isoptimum. At a pH much below 5 the aflinity of skins for the condensationproducts of methacrylic acid becomes so great that surface hardeningtakes place, while at a pH much above 5 the afllhity of the hide for thetanning agent is so decreased that the penetration is not sufllcientlyrapid for the best results.

The acidity of the polymerization products of methacrylic acid maysuitably be adjusted by the use of such alkalies as alkali metalhydroxides or carbonates, ammonium hydroxide, and organic bases, such asethanolamines. As has been observed, the pH of the hide should beconsidered in adjusting the pH of the tanning solution so that thetanning solution will be employed at an acidity within the rangeindicated. To assist in maintaining the tanning solution at about theoptimum acidity a suitable buffer salt such tanning agent has beeneffected the rate of tanning may be accelerated by lowering the pH ofthe solution. The pH to which the solution should be brought to effectcomplete tanning will vary considerably with the specific type of skinbeing treated. It will not generally be desirable to go to a pH below'about 3 since, at a higher hydrogen ion concentration, hydrolysis of theskin may take place. As a practical matter it will be found that the pHshould be lowered to from about 3.4 to 4.2, while more specifically inmany processes a pH of about 4 will be most satisfactory.

The lowering of the pH of the tanning solution may be eifected by theuse of any acidic material such as lactic, hydrochloric, sulfuric,acetic, phosphoric, sulfamic, and formic acids.

Considerations as to the type of apparatus to use, the time to beallowed for tanning, the temperatures of tanning, and the amounts oftanning agent are substantially those known to the art in connectionwith prior art tanning processes. The time allowed for tanning must besumcient to permit complete penetration of the tanning agent and the pHshould not be lowered too rapidly or there may be puckering and apparentover-tannage of the grain surface. The tanning'can best be effected atroom temperature say 20 to 30 C., but temperatures somewhat higher thanthose used by the art can successfully be employed if desired. The exactamount of the tanning agent to use can best be determined by a fewsimple tests under the specific conditions of the process selected andwith the specific skin to be treated. In general, enough of the tanningagent must be used to effect the tanning desired but unduly largeamounts should not be used as they will not. be taken up by the leatherin a reasonable period of time.

The leather obtained according to the preferred processes of thisinvention is white but it may if desired be treated with white pigmentsor may be colored during or after the tanning process by the inclusionof any of the common leather dyes and colored pigments.

While the polymerization products or methacrylic acid are valuabletanning agents in their own right, they may if desired be used togetherwith any other tanning agent or tanning assistant heretofore known tothe art. They may. for instance, be used in conjunction with vegetabletanning materials such as quebracho extract, or with synthetic tanningagents containing sulphonic acid groups. Additional fullnessf' orplumpness may be obtained by treatment with aluminum salts such asaluminum sulfate.

Leather produced according to the processes of this invention may besubjected to any of the various kinds of finishing treatmentscustomarily used.

My invention will be better understood by reference to the followingillustrative examples. Parts are given by weight.

Exsurra 1.-POZymethacrylic acid-water polymerized Nine hundred parts ofwater was placed in a water-jacketed stainless steel kettle equippedwith eflicient stirring apparatus, and was heated to 87 C. A solution of1.5 parts of benzoyl peroxide in one hundred parts of methacrylic acidwas then added to the water with vigorous agitation. Heating wascontinued to about 90 C. when a vigorous exothermic polymerizationreaction began. The temperature rose .to about 96 C. without furtherheating. Stirring was continued for a total of one and three-quartershours. The product was a fine gelatinous precipitate suspended in theaqueous medium. The reaction mixture was then cooled to 30 C. by runningcold water thru the jacket. On cooling, the polymer dissolved in thewater to form a colorless, somewhat viscous solution.

A tanning solution was made by dissolving 5.2

parts, dry weight. of the .product produced as above in 44.8 parts ofwater and adding this solution to 2.5 parts of salt and 5.0 parts ofsodium acetate dissolved in 100 parts of water. The pH of this solutionwas adjusted to 5.0 by the addition of a suitable amount of sodiumhydroxide. Fifty parts by weight of drained calfskin, which hadpreviously been pickled in a solution containing 1.5 per cent sulfuricacid and 10 per cent salt (NaCl), was placed in a tanning drum and 157.5parts of the above-described tanning solution was added. The pH of thetanning solution at this point was 5.0.

The hides were drummed for three hours after which the pH was found tobe 4.7. Acid additions were then made, 3 parts of a 3.6 per centhydrochloric acid solution being added each fifteen minutes withdrumming between each addition until six such additions had been made.At the end of this time, the pH of the liquor in the drum was 4.15. Thecalfskins were then removed from the drum, washed, and dried.

It was found that by the above-described treatment the calfskins hadbeen converted into white leather of excellent quality.

Following the procedure of this example, skins were successfully tannedusing successively larger amounts from 4 per cent to 10 per cent ofpolymethacrylic acid based on the drained pickled weight of the skins.Using 4 per cent of the polymethacrylic acid the tanning liquor wasexhausted while with 10 per cent of polymethacrylic acid presentcomplete exhaustion was not obtained. Larger or smaller amounts could beused if desired. Under the conditions of this example and with calfskinit appears that about 5 to 6 of suitable amounts of sodium hydroxide.

per cent of polymethacrylic acid based on the drained pickled weight ismost satisfactory.

Exsueu: 2.Polvmethacrylic acid-toluem polymerized A mixture of 0.9 partof benzoyl peroxide, 0.225 part of water, and 100 parts of methacrylicacid was added to 540 parts of toluene. The mixture was heated rapidlyto 100 C. in a vessel equipped with reflux condenser with rapid initialstirring to insure solution of benzoyl peroxide. A vigorous exothermicpolymerization .began at about 100 C., requiring cooling of the vesselwith wa ter to moderate the reaction. The reaction ves sel was held atabout 100 C. for five hours. The product was isolated as a fine whitepowder by centrifuging and air drying.

A tanning solution was made by dissolving 92 parts by weight of theabove toluene-polymerized polymethacrylic acid in 818 parts of water,adding 17.25 parts of sodium hydroxide dissolved in parts of water, anddiluting with 330 parts 215 water. The pH of this tanning solution wasEleven hundred and fifty parts of drained calfskins which had previouslybeen pickled in a solution containing 1.5 per cent 'sulfuric acid and 10per .cent salt were placed in a tanning drum, and a solution'containlng46 parts of salt in 575 parts 01 water was added. The skins were drummedfor ten minutes. The 1412.25 parts of tanning solution prepared as abovedescribed was then added to the tanning drum and the hides were drummedfor one hour and allowed to stand in the tanning'solution for sixteenhours without drumming.

Combination of the tanning agent with the hide was then effected byadding 30 parts by weight of 100 per cent lactic acid as a 10 per centsolution at intervals over a period of about six hours of drumming.These lactic acid additions efiected a lowering of the pH to 3.78. Thehides were then washed and dried.

It was found that the calfskins had been converted into white leather ofexcellent quality.

1 ExAuPLn 3.- -Polz' methacrylic acid-ultraviolet polymerizedMethacrylic acid was placed in a closed glass vessel and exposed toradiation from a mercury vapor are for five days, at which time thereaction product was a hard, glassy, semi-opaque solid. It was solublein dilute ammonia solution.

solution was placed in a tanning drum, and there was added twenty-fiveparts of drained goatskin which had previously been pickled with asolution containing 1.5 per cent sulfuric acid and 10 per cent salt andthen de-pickled with sodium acetate and washed.

The skins were drummed for four hours and allowed to stand in thetanning solution for an additional sixteen hours without agitation.Small amounts of a 10 per cent solution of lactic acid were then addedwith drumming between each addition until the pH had been lowered to3.6. The skins were then washed, fatliquored with a commercialfatliquor, and dried.

It was found that the goatskins had been converted into white leather ofexcellent quality.

This

and the solution filtered over kieselguhr.

EXAMPLE 4.Partially hydrolyzed methyl methacrylate Twelve parts ofmethyl methacrylate polymer was dissolved in ninety parts of benzene.The solution was heated to the boiling point in a vessel equipped with areflux condenser. A solution of eighteen parts of potassium hydroxide inseventy-two parts of absolute ethyl alcohol was slowly added. Thesolution turned dark but remained clear. After forty-eight hours furtherefiuxing, a gelatinous precipitate was formed. The alcohol and benzenewere decanted and a solution of forty-eight parts of potassium hydroxidein one hundred ninety-two parts of water was added to the residue in thereaction vessel. The precipitate slowly dissolved on heating. Theresulting solution was refluxed for seventy-two hours and then acidifiedto Congo red with concentrated hydrochloric acid. The resultingprecipitate was redissolved in a small amount of 5 per cent aqueouspotassium hydroxide solution The product was reprecipitated by droppingthe solution slowly into a large volume of boiling concentratedhydrochloric acid, filtered oil rapidly, and air dried overnight.Analytical data indicated that hydrolysis of the carbomethoxyl groups tofree carboxyl groups was 58 per cent complete.

A tanning solution was made by dissolving three parts by weight of thisproduct in one hundred parts of water containing a few drops ofconcentrated ammonium hydroxide. This solution was then adjusted to pH6.4 by addition of hydrochloric acid.

To this solution contained in a tanning drum was added twenty-five partsby weight of drained goatsklns, which had previously been subjected topickling and de-pickling, as described in Example 3, together with fiftyparts of 5 per cent sodium chloride solution. The skins were drummed forfour hours, and lactic acid additions were then made at intervals ofone-half hour'with drumming between each addition until the pH had beenlowered to 3.7. The skins were then removed, washed, fatliquored with acommercial fatliquor and dried. They were found to have been convertedinto white leather of good quality.

EXAMPLE 5.-Methacrylic acid-styrene interpolymer Five parts of styrene,thirty-five parts of methacrylic acid, and 0.5 part of benzoyl peroxidewere added to two hundred twenty-six parts of toluene and placed in avessel equipped with a reflux condenser. The reaction mixture wasrapidly heated to 90 C. A vigorous exothermic polymerization reactiontook place. The reaction mixture was heated at 90-100 C. forsix hours.It was then cooled and filtered. The product, isolated as a fine whitepowder, was dried under vacuum. I

A tanning solution was made by dissolving ten parts by weight of thisinterpolymer in two hundred parts of water containing a few drops ofconcentrated ammonium hydroxide. This solution was then adjusted to pH 5by addition of lactic acid.

To this solution contained in a tanning drum was added twenty-five partsby weight of drained calfskins which had previously been subjected topickling and depickling as described in Example 3. The skins weredrummed for one and onehalf hours, and lactic acid additions were thenmade at intervals of one-half hour with drumming between each additionuntil the pH had been lowered to 3.6. The skins were then removed,washed and dried, and were found to have been converted into whiteleather of good quality.

EXAMPLE 6.Methacrylic acid-methyl methacrylate interpolymer One part ofmethylated starch was dissolved in five hundred parts of water at 80 C.in a water jacketed vessel equipped with reflux condenser and anefficient stirring apparatus. A solution of one part of benzoyl peroxidein 87.5 parts of methacrylic acid and 12.5 parts of methyl methacrylatewas then added to the water solution with efiicient agitation. Avigorous polymerization reaction began. The reaction mixture was heatedto 92.5 C. with vigorous stirring for one hour and then cooled. Theproduct was a granular powder suspended in the aqueous medium. Thereaction mixture was cooled below 70 C. and 40.7 g. of NaOI-I dissolvedin 359.3 parts of water was added. Stirring was continued until a smoothviscous solution was obtained.

A tanning solution was made by dissolving in one hundred thirty-fourparts by weight of water ten parts, dry weight, of the aboveinterpolymer. The pH of the solution was adjusted to 6.0 by addingsodium hydroxide. A batch of skins was prepared for tanning by drummingfor ten min utes in one hundred parts by weight of drained pickledcalfskins with a solution containing four parts of sodium. chloride in50 parts of water. The above-described methacrylic acid-metha-- crylatesolution was then added to the tanning drum, and the hides were drummedfor three hours. Lactic acid was then added in small portions withdrumming until the pH had been lowered to 3.85. The skins were thenremoved from the drum, washed, and dried. It was found that thecalfskins had been converted to white leather of good quality.

acidic methacrylic acid polymers suitable for use in the presentinvention may be made in a number of ways. The polymerization may beeffected by any of several well-known procedures, such asat elevatedtemperatures with benzoyl peroxmers prepared by a variety of methods, asshown above, but it is to be observed that in the case of polymers ofvery high molecular weight, as judged by solution viscosity, the rateand degree of tanning maybe decreased somewhat, probably becausepenetration of the agent into the skin is impaired. Very low polymersalso should be avoided.

In addition to the polymers of the examples, the following illustrativeinterpolymers are suitable for use according to the present invention.All of these polymers, with the exception of the interpolymer ofmethacrylic acid with methacrylamide, which is a yellowish friableresin, are obtained as white powders of varying degrees of water andalkali solubility. All of them have been tested in accordance with themethods given With difierent As is apparent from the foregoing examplesthe 6. Leather'which comprises a reaction product of polymethacrylicacid and skin substance.

in the examples. and in each instance white leather of good quality wasproduced.

Methacrz llic acid interpolumers 5. Leather which comprises a reactionproduct of an acidic methacrylic acid polymer and skin substance.

ing in water forming a tanning solution with a pH of about 4.8 to 5.7.

GEORGE D. GRAVES.

' Proportions 5 a gggggg gg Method of preparing interpolymerSolubilities Original By analysis Diethyl iumarate-- 15% toluenesolution 01 monomers heated at 50:50 83:17 Readily soluble in water.

- 100:1 C. for 16 hrs. with 1.33% benzoyl per- Butadiene25%solutionoimonomersinmixtureoi'l4parts 80:20 Part1 soluble in water.

of toluene and 1 part carbon tetrachloride Read y soluble in dilutesodium heateddior 24 hrs. at 65 C. with 1% benzoyl hydroxide.

Vinyl acetate 153315116119 solution of monomers heated at 75:25 83516.5Soluble in water.

90-103:1 BC. [or 30 min. with 1.25% benzoyl Unsym dichloretliylene l5 gtoluene solution of monomers heated at 75:25 86:14 Partly soluble inwater.

90-100 C. for 15-16 hrs. with 1.25% benzoyl Readily soluble in dilutesodium peroxide. hydroxide.

Indene 15% toluene solution of monomers heated at 87.5:12.5 9413 Partlysoluble in water.

I 9011127" 0. for 1 hr. with 1.25% benzoyl perg Rilaladgixisioguble indilute sodium Vinyl chloride 25% toluene solution of monomers heated in80:20 Diflqcultiy soluble in water.

autoclave at65 C.for 24 hrs. with 1.0% ben- Readily soluble in dilutesodium zoyl peroxide. hydroxide.

Vinyl chloroacetate benzene solution of monomers heated 26.5 :50 72:28Insoluble in water. Soluble in hrs. at C. with 0.5% benzoyl peroxide.alkali.

Methecrylamide 15% toluene solution of monomers heated 16 :25 74:26Diflicultly soluble in water.

hrs. at -i00 0. with 1.25% benzoyl per- Readily soluble in dilute sodiumoxide. hydroxide. 25

Methyl methacry1ate As in Examplefl 75:25 Soluble in dilute alkali.

Methyl methacrylate-. AsinExampleG 62.5:37.5 Soluble in dilutealkali.

' In both columns, the percentage of methacrylic acid is given first.

Also suitable for use in the present invention '7. In a process for thetanning of skins, the 30 are the acidic polymers obtained by partial orstep comprising subjecting a skin to the action complete hydrolysis ofpolymers or'interpolymers of a methacrylic acid-methyl methacrylate inofmethacrylic acid derivatives, such as esters terpolymer. and amides, andof methacrylic nitrile. The use 8. Leather which comprises a reactionproduct of one such polymer is illustrated in Example 4.v of amethacrylic acid-methyl methacrylate in- 35 I While I have shown certainspecific illustrative terpolymer and skin substance.

processes and compositions, it will be understood 9. In a process forthe tanning of skins, the that without departing from the spirit of myinstep comprising subjecting a. skin to the action vention one skilledin the art may readily devise of a methacrylic acid-styreneinterpolymer. numerous tanning solutions and processes eml0. Leatherwhich comprises a reaction prod- 40 ploying polymerization products 01methacrylic uct of a methacrylic acid-styrene interpolymer acid. andskin substance.

I claim: 11. A tanning composition comprising a par- 1. In a process forthe tanning of skins, the tially neutralized acidic polymerizationproduct step comprising subjecting a skin to the action of methacrylicacid, the tanning composition upof an acidic polymerization product ofmethaon dissolving in water forming a tanning solucrylic acid. tion witha pH of about 4.8 to 5.7.

2. In a process for the tanning of skins, the 12. A taming compositioncomprising a parstep comprising subjecting a skin to the action tiallyneutralized polymethacrylic acid, the tanor polymethacrylic acid. v ningcomposition upon dissolving in water form- 0 3. In a process for thetanning of skins, the ing a tanning solution with a pH of about 4.8 tostep comprising subjecting a skin to the action 5.7. 1 of an aqueoussolution of an acidic polymeriza- 13. A tanning composition comprising apartion product of methacrylic acid at a pH of about tially neutralizedmethacrylic acid-methyl metha- 4.8 to 5.7, and then increasing theacidity. cryiate interpolymer, the taming composition 4. In a processfor the tanning of skins, the upon dissolving in water forming a tanningsostep comprising subjecting a skin to the action of lution with a pH ofabout 4.8 to 5.7. an aqueous solution of polymethacrylic acid at 14. Atanning composition comprising a para pH of about 4.8 to 5.7, and thenincreasing tially neutralized methacrylic acid-styrene interthe acidity.polymer, the tanning composition upon dissolv-

